1. Field of the Invention
The present invention generally relates to semiconductor technology and device designs, and more particularly to an organic-based thin-film transistor device and a method for producing the same.
2. Description of the Related Art
Thin-film transistors (TFTs) with active organic layers and polymer-based electronic components are emerging as an inexpensive alternative to silicon-based TFTs for some applications. The use of organic and polymeric materials provides two main advantages: First, organic-based devices can be produced using a simpler and less expensive fabrication process in contrast to the expensive equipment and processing associated with silicon processing. Second, it is possible to fabricate the devices on flexible plastic substrates due to the ability to process organic materials at lower temperatures, and due to the greater mechanical flexibility of organic-based components relative to inorganic materials such as silicon and conductive metals. However, despite considerable research and development effort, organic-based TFTs have not yet reached commercialization, at least, in part, due to relatively poor device characteristics of prior art organic TFTs.
Fabrication of an all-organic TFT requires various organic or organic/inorganic hybrid materials: semiconductors, insulators, and conductors. The conductor may be selected from conducting polymers such as polyaniline and poly(ethylene dioxide thiophene), and metal or graphite colloid particle-based inks. There are a variety of polymeric organic insulators that may be used, such as polyimide or PMMA for the semiconductor. Organic p-type (hole transporting) and n-type (electron transporting) materials are both known in the art and have been tested as the semiconductive channel in TFTs. Two relatively simple device structures which have been used are the top contact and bottom contact, as shown in FIGS. 1 and 2, respectively. Generally, these devices comprise a source 1 and drain 2. In the top contact (FIG. 1), the source 1 and drain 2 are on top of an organic semiconductor 3, whereas in the bottom contact (FIG. 2), the source 1 and drain 2 are embedded in the organic semiconductor 3. Below the organic semiconductor 3 is an insulator 4 in both the top and bottom contact devices. Embedded within the insulator 4 is a gate 5. The entire device (both the top and bottom contact devices) is disposed on a substrate 6.
FET mobilities are generally assessed in the top-contact geometry because applying the electrode materials on top of the semiconductor layer ensures intimate contact. It is desirable to use the bottom-contact configuration for some applications, but in this geometry, the contact between the electrode and semiconductor may be limited to a fraction of the vertical wall area of the electrode. This results in increased contact resistance. Problems with bottom-contact devices are well-known to those skilled in the art. A method to improve the bottom-contact geometry is to planarize the source and drain electrodes, which increases the area of contact to the organic semiconductor. A planar substrate allows for improved semiconductor films deposited by spin-coating or printing. However, topography of the bottom source and drain contacts in the prior art causes problems with printing the organic semiconductor, which would be remedied by using a planar substrate.
As mentioned, organic p-type (hole transporting) and n-type (electron transporting) materials are known in the art and have been tested as the semiconducting channel in TFTs. P-type materials include conjugated polymers and linear, conjugated molecules. Examples of p-type conjugated polymers include derivatives of regioregular polythiophene described in Bao and Lovinger, “Soluble Regioregular Polythiophene Derivatives as Semiconducting Materials for Field-Effect Transistors,” Chem. Mater., Vol. 11, pp. 2607-2612 (1999), the complete disclosure of which is herein incorporated by reference.
Examples of p-type conjugated molecules include pentacene, which has been extensively studied in TFTs, and further disclosed in U.S. Pat. Nos. 5,946,551; 5,981,970; and 6,207,472 B1; benzodithiophene dimers (Laquindanum et al., “Benzodithiophene Rings As Semiconductor Building Blocks,” Adv. Mater., Vol. 9, pp. 36 (1997); phthalocyanines (Bao et al., “Organic Field-Effect Transistors with High Mobility Based On Copper Phthalocyanine,” Appl. Phys. Lett., Vol. 69, pp. 3066-3068 (1996)); anthradithophenes (U.S. Pat. No. 5,936,259); and substituted and unsubstituted oligothiophenes, originally proposed in Gamier et al., in “Structural Basis For High Carrier Mobility In Conjugated Oligomers,” Synth. Met., Vol.45, pp. 163 (1991); the complete disclosures of which are herein incorporated by reference.
There are comparatively fewer n-type organic semiconductors. Examples include 3,4,9,10-perylene tetracarboxylic dilmides described in Struijk et al., “Liquid Crystalline Perylene Dilmides: Architecture and Charge Carrier Mobilities,” J. Am. Chem. Soc., Vol. 122, pp. 11057-11066 (2000); 1,4,5,8-naphthalene tetracarboxylic dianhydride (Laquindanum et al., “n-Channel Organic Transistor Materials Based on Naphthalene Frameworks,” J. Am. Chem. Soc., Vol. 118, pp. 11331-11332(1996)); 1,4,5,8-naphthalenetetracarboxylic dumide derivatives (Katz et al., “Naphthalenetetracarboxylic Dilmide-Based n-Channel Transistor Semiconductors: Structural Variation and Thiol-Enhanced Gold Contacts,” J. Am. Chem. Soc., Vol. 122, pp. 7787-7792 (2000)); and metallophthalocyanines substituted with various electron-withdrawing groups (Bao et al., “New Air-Stable n-Channel Organic Thin-film Transistors,” J. Am. Chem. Soc., Vol. 120, pp.27-208 (1998); the complete disclosures of which are herein incorporated by reference.
In general, circuitry using organic transistors has the potential of reduced power consumption and simplicity in the design. However, complementary circuitry using both organic N and P channel transistors are not common, for example, U.S. Pat. No. 5,625,199, the complete disclosure of which is herein incorporated by reference, teaches a technique to fabricate complementary circuits with inorganic n-channel and organic p-channel thin-film transistors. Additionally, U.S. Pat. No. 5,936,259, the complete disclosure of which is herein incorporated by reference, describes a switch based on a thin-film transistor design (TFT) using a fused ring organic compound as a semiconductor. Furthermore, U.S. Pat. No. 5,804,836, the complete disclosure of which is herein incorporated by reference, describes an image processor design which operates on an array of polymer grid triodes. Similarly, prior art disclosures also teach a 5-stage ring oscillator using copper hexadecafluorophthalocyanide as the n-channel material and oligothiophenel oligothiophene derivative as the p-channel material.
Two popular structures of an existing polymer thin-film transistor are shown in FIGS. 3(a) and 3(b). These structures have two major disadvantages. First, the corner thinning problem due to topography, and second, the most sensitive portion of the body element is exposed to process-induced contamination. The resulting devices have poor performance and inconsistent properties. Shown in FIG. 3(a) is the first typical structure of the polymer transistor. The source 11 and drain 12 are first patterned. Then, the body material 13 is deposited and patterned. The body 13 is a semiconductive polymer or oligomer, and it is applied to the surface of the source 11 and drain 12 islands by evaporation, spin-coating, dip-coating or printing, depending on the organic semiconductor used. The body material 13 is patterned in one of three ways: the most common method is by evaporation of the semiconductive material through a shadow mask.
The other two methods are printing (i.e., screen printing or inkjet printing) and using conventional lithography by first applying a protective coating over the semiconductor, then applying the photoresist, patterning, and etching. A brief thermal anneal may be needed, depending on which type of organic semiconductor material is used. The last step includes applying a protective coating to the semiconductor to passivate the devices from contamination. After patterning the body portion 13, the substrate is wet cleaned. The body surface, especially in the channel region, deteriorates due to the unwanted chemical reaction. After a thermal treatment, the body element 13 becomes thin around the corners 16, 17 of the source 11 and drain 12 due to reflow. Typically, semiconductors decompose before melting. The source/drain 11, 12 to body contact area are significantly reduced as a result of the corner thinning 16, 17 of the body element 13. Then, the gate material 14 is deposited after a thin insulating polymer 15 is coated on top of the body element 13 and the exposed source 11 and drain 12 regions.
Another common structure of the polymer TFT structure is shown in FIG. 3(b). The gate 314 is formed first, then an insulating polymer 315 is coated thereon. Again, the corner thinning presented at the corners 316, 317 of the gate 314 causes the possibility of shorting of the source 311 and drain 312 to the gate 314. After the source 311 and drain 312 are formed, the body element 313 is formed. In this case, since the body to channel interface is not exposed to the chemical, the resulting transistor yield and performance are better than those of the first transistor.
In both of the bottom-contact devices described above, there is a well-documented problem with ensuring good contact between the electrodes and the organic semiconductor. One approach to solving this problem has been to modify the surface properties of gold electrodes using thin, self-assembled monolayers, which improves wetting of the electrode by the organic semiconductor and which may also decrease the chance of delaminating. However, the topography of the bottom electrodes may still hamper film formation and reduce the contact area. Therefore, there is a need for a new and improved structure and method for producing a planar polymer transistor which does not have the problems inherent with the prior art devices.